Study On Treatment Of Phenolic Wastewater By Wet Catalytic Oxidation - Adsorption

With the wastewater discharge requirements becoming more stringent due to the demand for environmental protection, more attention has been paid to the treatment of phenolic wastewater, which is of high-toxic and refractory for traditional biological method. Generally speaking, a single treatment method is difficult to meet the needs of wastewater discharge requirements. The development of a combined process comprising a variety of water treatment techniques thus has become one of the research hotspots. In this work, the technique of wet catalytic oxidation combining with adsorption method was applied for the advanced treatment of simulated phenol wastewater. In this combined process, the phenol wastewater is catalytically degraded by Fenton-like system, following a successive adsorption process for the thorough removal of residual phenol contaminant.Fenton catalytic oxidation is extensively used for the degradation of refractory organic contaminant existed in the industrial effluents with the advantages of completely minimization of organic contaminants into environmentally-friendly micromolecules such as CO2 and H2O. Fe-C composite nanofibers (Fe-CNFs) were prepared by electrospinning polyacrylonitrile (PVP), iron (Ⅲ) acetylacetonate and dimethyl formamide (DMF) compound solutions, followed by thermal treatments of successive pre-oxidation and carbonization. The morphologies and crystallography of the composite nanofibers were characterized by SEM and XRD respectively. The Fe-CNFs samples were used as the catalysts for the degradation of phenol wastewater. The catalytic activities and long-term cycling stabilities of Fe-CNFs samples were evaluated by the depended of the removal of phenol and COD on temperature and time. The operation parameters of the Fenton catalytic oxidation was also optimized as well as the determination of the evolution of Fe species through the Fenton process by XPS patterns. However, it is evidenced that the existence of a certain amount of organic micromolecules in the effluents after undergoing the treatment of Fenton catalytic oxidation. In view of this reason, an additional advanced process of adsorption was adopted just following the Fenton treatment.Activated carbon fibers (ACF), which is widely used as an efficient adsorbent, are of remarkable merits of high specific surface area, narrow, pore size distribution and abundant uniform micropores which are accessible for the mass transfer of adsorbents. In a typical adsorption process, phenol-adsorbed ACF sample is subsequently subjected to the regeneration process to restore its adsorption capacity. The electrochemical oxidation was chosen as the regeneration method of phenol-adsorbed ACF sample in this study. Three-phase electrochemical batch reactor has been designed and the electrochemical regeneration behavior has been studied in detail. The reusable adsorption capacities of the samples were estimated by determining the regeneration efficiency (RE) and COD removal during the regeneration process. The main conclusions are as follows:(1) The prepared AAI-PVP composite nanofibers (15wt%、30wt% and 50wt%) by electrospinning exhibited a continuous, aligned fibrous morphologies with the fiber diameter in the ranges of 200-900 nm. Due to the results of TG analyses in O2 and N2 atmospheres, the pre-oxidation temperature for AAI-PVP samples was chosen as 250 ℃ as well as the carbonization of the samples were carried out at 500 ℃,550 ℃ and 600 ℃ respectively. The incorporation of AAI into PVP/DMF solutions resulted in the reducing of the solution viscosity which is answer for the decrease of fiber diameters. Besides, the shrinkages of fiber occurred during pre-oxidation and carbonization, leading to a diminution of fiber section area, as well. XRD results showed that iron oxide transformed into iron carbide in composite nanofibers with increasing carbonization temperature. The obtained composite nanofibers samples, which is denoted as Fe(15)-CNFs, Fe(30)-CNFs and Fe(50)-CNFs respectively, can be used as the heterogeneous Fenton catalysts for the degradation of phenol wastewater.(2) Fe-CNFs samples, fabricated at the carbonization temperature of 500℃, showed an attractive catalytic performance in Fenton oxidation process with a phenol removal efficiency of 92.76% and a COD removal efficiency of 79.43% respectively. The existence of an induction period in the initial stage of phenol degradation tests suggest a poor wetability of the Fe-CNFs samples which inhibit the interaction of phenol in solution and Fe(II) species on the surface of the catalyst. An elevated reaction temperature lead to a decrease of the induction period and the reaction rate would be improved consequently. By comparison with the initial Fe doping amount on Fe(50)-CNFs sample, around 5% Fe leaching (i.e.,3.56 mg/L) was demined after the Fenton oxidation process.(3) XRD measurement revealed that Fe3O4 in Fe(50)-CNFs is the unique Fe species throughout the Fenton oxidation process. An increased molar ratio of Fe2+/Fe3+ were observe by XPS results of the Fe(50)-CNFs. The O=C-O functional group was found on Fe(50)-CNFs after Fenton oxidation suggesting an improvement of the wettability for Fe(50)-CNFs sample, which may be the evidence of the presence of the induction period and the Fe release during the degradation process.(4) The adsorption tests show a good adsorption performance for commercial ACF samples with the adsorption time of 30min reaching up to adsorption/desorption equilibrium. There existed little phenol remained in the solution and the COD was 10.03mg/L after adsorption process.(5) In our work, a novel β-PbO2 anode was prepared with long-term cycling life, excellent catalytic performance and its oxygen evolution potential is 1.4V. The results showed that external diffusion has a significant impact on RE, and the modified three-phase electrochemical batch reactor could effectively reduce the mass transfer resistance, resulting in an improvement of 15% RE.(6) The usage of NaNO3 electrolyte could contribute to the improvement of COD removal. The RE and COD removal were increased at higher current density. In view of RE and EUR, a current density of 12 mA/cm2 and temperature 30 ℃ were chosen in this work for the achievement of a good performance and low energy cost as well.(7) Ultrasound field applied in the electrochemical regeneration reactor can effectively improve both mass transfer and reaction, and consequently increase the RE of ACF samples.